From the desk of Gus Mueller
One of things we all learned in our early days of audiology training is how to use the configuration of the audiogram to assist us in determining the etiology of the hearing loss. You know how it goes—a symmetrical gradually downward sloping hearing loss in an older individual often indicates presbycusis, a notch in the 3000-4000 Hz region often is associated with noise exposure, and so on. It's probably natural then that as years go by and we see hundreds or thousands of patients, we tend to start categorizing patients by their audiograms, and sometimes use this single measure for making treatment decisions. While this process can be helpful, it also can get us into trouble.
Consider the 72-year-old male that you are fitting with hearing aids today. He has a symmetrical loss starting at 30 dB in the lows sloping to 70-75 dB in the highs; his word recognition scores are pretty good. Nothing unusual, and in fact you fit a guy with hearing aids just last week who was about the same age and had an almost identical hearing loss. He loves his new hearing aids, so today's patient should be a slam dunk, right? But does today's patient have the same loudness judgments? The same ability to understand speech in background noise? The same central processing and cognitive skills? The same motivation to use hearing aids? The same lifestyle or listening needs? Maybe this is a really different patient than the one you saw last week, but just happens to have the same pure-tone thresholds.
Whether it relates to diagnostic or rehabilitative audiology, looking beyond the audiogram is essential. To remind us of the different areas where this can be helpful, we have brought in a 20Q guest who is both a clinician and an applied researcher, who has experienced many of the factors related to "patient individualization" first hand.
Dr. Kevin Munro has a background in medical sciences and obtained an MSc (Distinction) and PhD in Audiology at the University of Southampton. He also has a Diploma in Management Studies. He is currently a Professor at the University of Manchester, where he has worked the past ten years. As you know from his many presentations and publications, his audiology interests are diverse, and include the assessment and rehabilitation of hearing and balance disorders in adults, the assessment and habilitation of hearing (including cochlear implants) in children, and research related to plasticity of the auditory system.
Kevin is an active member of the British Society of Audiology; he served as editor of British Society of Audiology News and is presently the Chair of this organization. The Society has awarded Kevin the Thomas Simm Littler prize for his contribution to research in audiology, and the Jos Millar shield.
Whether it's RECDs for infants, or brain plasticity in older adults, over the years Kevin has continually reminded us that it's important to examine individual differences when working with all patients. At last years' annual British Academy of Audiology meeting, he was honored as the John Bamford Lecturer, and gave the presentation titled: Individualized hearing healthcare across the lifespan. Our 20Q with Dr. Munro is based in part on that lecture, which can be viewed here.
Gus Mueller, Ph.D.
Contributing Editor
May 2012
To browse the complete collection of 20Q with Gus Mueller articles, please visit www.audiologyonline.com/20Q
1. I'm not sure what you mean by "individualized practice." Isn't that what we are already doing when we conduct audiometric testing?
Yes and no. Health care practices have generally centered on the outcome of large epidemiological studies. A large scale study may show that "Treatment X" is better than "Treatment Y" but this doesn't take into account individual differences within a population. For example, there may be some people, with the same audiometric configuration, who are better with hearing aid X (or signal processing feature X) and others with hearing aid Y. Individualization is a hot topic in many areas of healthcare.
2. OK, so let's start with pure-tone hearing thresholds. You've got to admit that these are individualized?
It is certainly true that we rely on pure tone hearing thresholds a lot in clinical practice, but they don't tell us everything. For example, a common approach is to compare the hearing thresholds from one patient with what we expect, on average, from someone of a similar age. Did you know that if you take a group of people around age 70 and compare their hearing thresholds, you'll get a range of about 60 dB at most audiometric test frequencies? For example, at 1 kHz, the median hearing threshold level will be around 25 dB but it could range from 5 dB to 70 dB (Davis, 1995). We don't all live in exactly the same environment or have exactly the same genetic susceptibility to hearing loss; otherwise, we would all acquire the same hearing loss at the same age. Understanding why we differ is an important question because it may influence what we do in clinical practice.
3. Can you give me a specific example?
Recent data from Christine Tan and colleagues (Tan, McFerran, & Meddis, 2011) appear to show that, despite similar hearing levels, adults with and without tinnitus can be differentiated on measures of basilar membrane compression and frequency selectivity. This could potentially help us differentiate individuals and lead to more personalized treatment options that optimize the benefit of rehabilitation. Also, we tend to group everyone with an age-related hearing loss into the same category but we know that cochlear hearing loss is not a single physiological entity. For example, it can result from metabolic changes in the cochlea and not just loss of outer hair cells (Schmiedt, 2010). It is in its early days but this distinction could be important because it could influence how we prescribe amplification. Patients with a metabolic hearing loss may not have a reduced dynamic range like those with outer hair cell damage. This means that they may benefit from a different hearing aid prescription.
4. Are you saying that we may need different hearing aid prescription targets, even if two patients have the same hearing loss?
Don't sound so surprised. Both DSL5.0 and NAL-NL2, the most commonly used prescription approaches, are already moving in that direction. Researchers at NAL have shown that, compared to new adult hearing aid users, experienced users prefer more gain. Also, adult males prefer slightly more gain than females. Therefore, the prescribed gain for an experienced male user can be significantly higher than a new female user (Dillon, Keidser, Ching, Flax, & Brewer, 2011).
5. What about those of us using DSL?
Yes, amplification targets for this prescriptive method also differ depending on a number of factors, such as if the patient is an adult or a child (Moodie, Scollie, Bagatto, & Seewald, 2006). Also, don't forget that DSL has a longstanding record of personalising the prescription of hearing aids by taking into account the large individual variation that occurs in the occluded ear canal. This difference is now widely recognized as being important and other prescription approaches now incorporate this into their fitting methods.
6. It sounds like you might be talking about RECDs, right?
Yes, the dimensions of the external ear vary in much the same way as we differ in terms of other physical dimensions and this affects the amplified sound level in the ear canal. For example, you wouldn't dream of fitting everyone with the same size of shoe, would you?
7. No, I guess that's true.
In much the same way, we now recognize the importance of individualizing the infant hearing aid fitting by accounting for these differences in physical dimensions. Actually, just like our size of our right and left foot, we do seem to be fairly symmetrical so it may not always be necessary to make RECD measurements on each ear of the individual. We recently conducted a study in which we compared the RECD values from the right and left ear of hearing aid users (Munro & Howlin, 2011). Our sample included adults, school-age children and pre-school children. In around 80-90% of cases, the difference between ears was generally less than 3 dB at audiometric frequencies from 0.5 to 4 kHz. This finding is very similar to what we reported way back in 2005 in a group of cooperative adult participants (Munro & Buttfield, 2005). This leads us to think that (when there is no occluding wax and normal middle ear function), the difference between the right and left ear is small and probably not clinically significant. Another way of looking at this is if you can only measure the RECD from one ear of a patient then it's probably better to enter the same values into the fitting software for both ears than to rely on average default values.
8. Is there any reason why I need to think about RECDs when I fit hearing aids for adults?
There most certainly is. The high inter-subject variability in the acoustic properties of the external ear is not restricted to infants and young children. For example, Saunders and Morgan (2003) reported that the distribution of eardrum SPL for a fixed dial level at 1 kHz was 40 dB in a group of 1814 adults. Even allowing for some experimental error, this demonstrates that it is inaccurate to assume that, across ears, a given HL signal will result in the same ear canal SPL. Audiometric transducers such as the supra-aural and the insert earphone are based on calibration data which aim to give 0 dB HL in a normal hearing adult. However, this is not the case in the patient who does not have an 'average' adult external ear canal. This problem can be eliminated if the reference point for measuring SPL is the ear canal since the threshold of hearing will be the same regardless of the audiometric transducer and the external ear acoustics. As you know, today we conduct probe-mic verification using speechmapping and we use ear canal SPL as the reference. This allows us to correct for differences in ear canal dimensions across adults, although few clinicians use this option. Without this correction, you could have the wrong hearing aid programming simply because the patient did not have an average RECD.
9. Now you've got me thinking. Does different hearing aid technology need to be individualized too?
Maybe. One area that has generated some research interest is compression release times. As you know, compression is used to compensate for the effects of loudness recruitment but it can cause the gain to change relatively slowly (AVC) or relatively quickly (syllabic compressors). An advantage of AVC is that speech can be delivered at a comfortable level and the temporal envelope of the speech is hardly distorted. An advantage of syllabic compression is that it can restore loudness perception to normal, and it can do this in a frequency-specific manner (if the hearing aid has multiple channels). When comparing benefit of different compression time constants in groups of hearing aid users, there is not a lot of evidence of a clear winner, but there is substantial individual variability. In recent years, researchers have been trying to understand this variability by investigating predictors of benefit for different compression times. This work could potentially lead to a new clinical protocol that we could use to individualize this aspect of our fitting process.
10. I could see how this could be useful. What sort of predictors are you talking about?
Investigating cognitive abilities (the brain processes required to perform specific tasks) might be important. We already know that individuals with a moderate hearing loss perform better in difficult listening situations if they have a high cognitive ability. We also know that performance with cochlear implants is related to cognitive ability. Gatehouse and colleagues were one of the first to investigate cognitive ability and compression times in hearing aid users (Gatehouse, Naylor, & Elberling, 2006). They used a visual monitoring test where individual letters appeared on a screen and the task was to watch out for a sequence making a 3-letter word (this involves memory and speed of processing).
11. Did they find anything interesting?
They showed that patients who performed well on this task were able to benefit most from fast release times. A number of studies have since been published on this topic too (Lunner & Sundewall-Thoren, 2007; Foo, Rudner, Ronnberg, & Lunner, 2007; Cox & Xu, 2010). The individual studies showed a relationship between compression times and cognition but they didn't all agree on the details. Some of the discrepancy probably depends on the nature of the cognitive task, the listening test, characteristics of masker, etc.
12. How many of these lab-based findings are relevant to real-world benefit?
OK, I think you are referring to the distinction between efficacy (how well something works in a clinical trial) and effectiveness (how well a treatment works in the real world). The Cox and Xu (2010) study deals with the issue of individualizing compression times and real-world benefit. Almost all their participants had a preference for either a slow or fast release time (around two-thirds preferred a slow release time) and their self-reported benefit was higher when using their preferred release time. This suggests that finding a way to select an appropriate release time when the hearing aid is being fitted could help maximize individual benefit. I believe that more work needs to be carried out on this topic before we can use these data for individual management.
13. Any other potential predictors besides cognitive ability?
Well, the ability to 'dip listen' might be relevant. This is where a person can take advantage of dips in fluctuating background noise when trying to follow and understand speech. There is some evidence that the benefit obtained from dip listening is related to the ability to process the temporal fine structure of sound. Brian Moore (2008) has proposed that a test of the ability to process temporal fine structure might be a useful way for individualizing compression times. At the end of the day, we may need to measure a variety of factors in order to predict the most appropriate hearing aid settings and intervention strategy for each patient.
14. You mentioned Gatehouse. Is that the same Stuart Gatehouse who developed and validated the Glasgow Hearing Aid Benefit Profile?
That's right and the GHABP is relevant to our discussion. Self-reported outcome measures (or questionnaires) are now used much more routinely in clinical practice, and a vast array are available. One way they can be categorized is the extent to which all patients are asked about the problems they experience in the same listening environments or, importantly, in the environments that are relevant to them. That's an important distinction. The GHABP is a good example of the latter (Gatehouse, 1999a). The COSI is another good example (Dillon, James, & Ginis, 1997). The customization or individualization of self-report questionnaires is an important step forward in our practices.
15. And it's a step forward because . . .?
Well, individual relevance is based on coaxing the patient to identify the environments, or listening situations, where it is important for them to hear well. There is good evidence that this is a helpful thing to do (Gatehouse, 1999b). After all, why measure benefit for listening situations that are not important to the patient? The GHABP allows the patient to identify up to 4 environments (in addition to 4 standard environments which, incidentally, can be ignored if these are not relevant) where it is important for them to hear well. When the GHABP was being developed, every patient was able to identify at least one listening situation that was important to them (and around 75% identified four additional listening environments). If used properly, the combination of scores on the pre- and post-fitting sections of GHABP can be used to tailor intervention to the individual. The COSI operates along similar lines and is very quick to administer in a busy clinic but, in my experience, most patients score quite well. I therefore use it mainly to alert me to the poor performers.
16. You started out by discussing the pure tone audiogram. My colleagues tell me that this is of limited value for specific conditions such as auditory neuropathy spectrum disorder (ANSD). Is this correct?
Yes, ANSD is worth mentioning here because identification and management of babies and young children with this clinical diagnosis is challenging (Northern, 2008). Our standard hearing assessment methods are of limited value because we are unable to estimate threshold levels from the auditory brainstem response in these babies and, in any case, the relationship between hearing thresholds and functional ability is very poor. The range of outcomes is extremely wide yet decisions have to be made about intervention strategies including the provision of hearing aid(s) or cochlear implant(s). Procedures that will help understand the heterogeneity and lack of predictability regarding outcome will help us to individualize management.
17. Any procedures on the horizon?
Identifying the site of the dysfunction in each individual with this clinical diagnosis could be very helpful. For example, a mutation of the otoferlin gene is known to affect the junction, or synapse, between the inner hair cell and the auditory nerve. This information might aid decisions about the provision and outcome of a hearing aid or cochlear implant. Although obligatory cortical auditory evoked potentials are not yet a standard clinical measure in babies, there is some evidence that they show promise as an objective clinical tool for predicting speech recognition and functional outcome in young children with ANSD (Rance, Cone-Wesson, Wunderlich, & Dowell, 2002; Pearce, Golding, & Dillon, 2007; Sharma, Cardon, Henion, & Roland, 2011).
18. Any other possible predictors of outcome that may be relevant when working with patients who have a hearing loss?
Brain plasticity may well be important. It is well known that the brain can reorganize when the sensory input changes both in adults and in children. Investigating the perceptual consequences of these changes is a hot topic (Munro, 2010).
19. I agree it's an interesting topic, but how does it relate to my day-to-day practice?
That's an area that we're looking into, but here is one example related to hearing aid fittings. We have observed changes in loudness discomfort level (LDL) in adults with age-related hearing impairment who have been wearing a monaural hearing aid (Munro & Trotter, 2006). The results show that the LDL are similar in both ears before the adults use the hearing aid, but after hearing aid experience, they tend to become more tolerant of loud sounds in the ear that has had the hearing aid. So, for example, the LDL for a 1000 Hz tone may be 90 dB HL in both ears before hearing aid use but increases to 95 dB HL in the ear that has been stimulated by the hearing aid. Before going any further, I should point out that some researchers have observed no (or only very small) changes after hearing aid experience. I have two comments. First, I am saying that it is possible to measure these changes in the laboratory but I don't yet know if they are necessarily big enough to make a difference to the hearing aid fitting. Second, our most recent study doesn't appear to show changes in adults who wear bilateral hearing aids (Hamilton & Munro, 2010). This may mean that it is the asymmetry caused by using one hearing aid that drives the adaptive plasticity.
We'll need to do more work in this area to help unravel these findings. We have many studies currently underway. For example, Piers Dawes in my research group is collaborating with Brent Edwards and colleagues from the Starkey Hearing Research Center on a study that is investigating brain plasticity and perceptual consequences in adults fitted with one versus two hearing aids. This study includes measures that may predict individual benefit. We have just finished data collection so we will be analyzing the findings very soon. I can't help but mention one preliminary finding that has caught our eye, even though it's not strictly related to individualizing clinical practice. We measured hearing aid use, via data logging, and have tentative evidence for a relationship between auditory acclimatization (or attentional tuning) and daily hearing aid use: the more the patient uses the hearing aid, the more likely there will be improvements in benefit over time.
20. Any final comments about how we will individualize clinical practice in the future?
We are in an era of personalized medicine where we are able to look at the entire genetic code within a cell. This is relevant to audiology because it has the potential to provide information about the risk of hearing loss to the individual, and how it can be prevented. For example, it may become possible to assess individual susceptibility to hearing loss from environmental factors such as noise and ototoxic drugs. This could inform preventative measures and treatment options for the individual. In the future, I think it is likely that individualizing hearing healthcare will be achieved from advances in a number of disciplines working in combination including audiology, hearing science, cognitive and health psychology, and genetics.
Acknowledgements
Thank you to Piers Dawes who provided helpful comments on an earlier draft.
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